The present invention relates to a method of making a semiconductor whose major component is silicon having crystalline property, more particularly, to a method of making a crystalline silicon semiconductor used in a semiconductor element such as a thin film transistor.
Conventionally, a thin film transistor (hereinafter, TFT) using a thin film semiconductor has been known. TFT is constituted by using a thin film semiconductor formed on a substrate. TFT is used in various integrated circuits, especially in an electrooptical device such as a liquid crystal display, particularly as a switching element at a respective pixel of a liquid crystal display device of an active matrix type or a driver element formed in a peripheral circuit portion.
Although it is simple and convenient that an amorphous silicon film is used as a thin film semiconductor of TFT, its electric properties are poor. It is preferable to use a silicon thin film having crystalline property for promoting the properties of TFT. To provide a silicon film having crystalline property, an amorphous silicon film is firstly formed and crystallized thereafter by heating it or by irradiating thereon an electromagnetic wave having high energy such as a laser beam.
However, the crystallization by heating needs to take a time period of 10 hours or more at a heating temperature of 600xc2x0 C. or more and accordingly, it is difficult to use a glass substrate. For example, the glass strain point of Corning 7059 generally used in an active type liquid crystal display device is 593xc2x0 C. and therefore, the heating at 600xc2x0 C. or more is problematic when area expansion of a substrate is considered. Further, the properties of the obtained crystalline silicon film are poorer than those of a film provided by a laser beam irradiation as follows.
It has been revealed that an element having a catalytic action accelerating crystallization of amorphous silicon is used to solve the problem as is disclosed in Japanese Unexamined Patent Publication Nos. 244103/1994, 244104/1994, 244105/1994, 244205/1994 and 296023/1994. That is, it has been revealed that the crystallization can be performed in a processing time of approximately 4 hours at 600xc2x0 C. or less, typically, 550xc2x0 C. by making an element of nickel, palladium, lead or the like adhere to an amorphous silicon film in a very small amount and heating it thereafter.
However, the catalyst element remains in such a process in the silicon film obtained in such a short period of time at such a low temperature and the properties of TFT using the silicon film are not preferable. Especially, it is the most serious problem in TFT that when a backwardly biased voltage (negative voltage in N-channel TFT, positive voltage in P-channel TFT) is applied on the gate, the absolute value of a drain current (off current or leakage current) is large and the value is considerably dispersed among respective elements.
Especially, the large off current causes a serious problem when the silicon film is used in a switching transistor of a pixel electrode in an active matrix type liquid crystal display device. When the off current of a thin film transistor arranged at a pixel electrode is large, the pixel electrode cannot hold electric charge during a predetermined time period which causes flickering of a screen and an obscure display.
In view of the current status it is an object of the present invention to provide a method of making a crystal silicon semiconductor film capable of reducing the off current of TFT and reducing the off current value of each element and its dispersion by adopting a step of crystallizing a silicon film by using a catalyst element promoting crystallization of silicon, especially, a method of making a crystal silicon semiconductor capable of being treated at a low temperature and preferable to mass production.
To achieve the above-mentioned object the present invention provides a silicon film having crystalline property by using the following steps.
Firstly, a silicon oxide film is deposited on an insulating surface by various chemical vapor deposition (CVD) processes, for example, plasma CVD process or thermal CVD process. The film forming temperature in this step is 450xc2x0 C. or less, preferably, 300 through 350xc2x0 C. The deposition may be performed by the plasma CVD process using, for example, tetraethoxysilane (TEOS) and oxygen or mono-silane (SiH4) and dinitrogen monoxide (N2O) or the thermal CVD process using mono-silane and oxygen.
An amorphous silicon film is deposited by various CVD processes on the silicon oxide film which has been deposited as mentioned above. For example, the film forming temperature of 295 through 305xc2x0 C. is preferable in obtaining an amorphous silicon film by the plasma CVD process with mono-silane as raw material. However, it is necessary to form the amorphous silicon film without bringing the silicon oxide film into contact with the atmosphere. That is, the formation of the silicon oxide film and the amorphous silicon film needs to be continuously performed. It is preferable for this purpose to use a film forming device (cluster tool) of a publicly-known multichamber system.
Thereafter, a single substance of a catalyst element or a compound containing the catalyst element promoting crystallization of the amorphous silicon film is formed on the amorphous silicon film in a layer, a film or a cluster. Hereinafter, a layer of a single substance of a catalyst element or a compound containing the catalyst element is called a catalyst layer. The method of forming a catalyst layer will be mentioned later.
Further, the inventors found that the most significant effect is achieved when nickel is used as a catalyst element. The other usable catalyst elements are Pt,Cu, Ag, Au, In, Sn, Pd, P, As and Sb.
Thereafter, a heating process is performed on the amorphous silicon film by which a portion or a total of the amorphous silicon film is crystallized. In the process of crystallization, when the catalyst layer does not cover the total face of the amorphous silicon film, not only a region which the catalyst layer covers is crystallized but the crystallization is progressed from the region to peripheral portions.
In the crystallizing step the amorphous silicon film is heated at a temperature of 400xc2x0 C. or more such that crystallization of the amorphous silicon film in which the catalyst element has been introduced is progressed. In a general glass substrate the heating temperature is 400xc2x0 C. to 750xc2x0 C. However, the heat resistant temperature differs by the kind of the glass substrate and accordingly, the upper limit of the heating temperature may be the strain point of glass. For example, the glass strain point is 593xc2x0 C. for Corning 7059 glass and 667xc2x0 C. for Corning 1737 glass.
Specifically, it is appropriate to determine the heating temperature as approximately 550xc2x0 C. in view of heat resistance and productivity of a glass substrate.
It has been clarified that the higher the heating temperature, the more improved is the crystalline property of the silicon film. Therefore, the silicon film is heated at a temperature as high as possible so far as the substrate can stand the temperature in case where the crystalline property of the silicon film is mostly preferred. In this case it is preferable to use a quartz substrate which can stand a temperature of approximately 1000xc2x0 C. For example, a quartz substrate can be heated at a temperature of approximately 800xc2x0 C. through 1000xc2x0 C.
The crystallization may more be promoted by irradiating a laser beam or an equivalent strong beam after the heating step. By adding this step portions which could not be crystallized in the previous step can be crystallized in which portions that have been crystallized in the previous step are used as nuclei.
The basic difference between the crystallization by the present invention and the conventional crystallization performed by irradiating a laser beam lies in that conditions determining the crystalline property are very severe in the conventional method since the amorphous silicon film is molten from a state of no crystals and thereafter crystallized. That is, when nuclei are not present, the cooling rate is a dominant factor determining the crystalline property in the crystallization process. However, the cooling rate considerably differs depending on the energy density of a laser beam and a temperature of the environment and necessarily, the optimum range of laser energy density is narrowed. If the energy is excessively high, the cooling rate from the molten state is excessively large bringing about an amorphous state. Further, if the energy is excessively low, the total of the film cannot be molten and amorphous portions remain.
Meanwhile, when nuclei are present, crystallization is facilitated and the dependency on the cooling rate is inconsiderable. Further, most of the film has been crystallized and therefore, an appropriate characteristic is guaranteed even if the energy density of the laser beam is low. In this way it is possible to stably provide a crystal silicon film having extremely improved crystalline property.
An incoherent strong beam, especially an infrared ray may be irradiated in a short period of time in place of the laser beam radiation. An infrared ray is difficult to absorb in glass and easy to absorb in a silicon thin film and accordingly, a silicon thin film formed on a glass substrate can selectively be heated which is preferable. A process of such an infrared ray in a short period of time is called rapid thermal annealing (RTA) or rapid thermal process (RTP).
As a method of forming a catalyst layer there are deposition process in which a vacuum device as in sputtering a single substance of a catalyst element or its compound is used and a deposition process in which a solution containing a catalyst element is coated on the surface of an amorphous silicon film in the atmosphere. Especially in the latter process the deposition can be performed reproducibly without capital investment. A detailed explanation will be given of the latter process as follows.
In the latter process an aqueous solution, an organic solution or the like can be used as a solution. In this specification xe2x80x9cinclusion of a catalystxe2x80x9d indicates that a catalyst is included as a compound or a catalyst is included in dispersion.
As the solvent containing a catalyst element a polar solvent of water, an alcohol, an acid, or ammonia can be selected. In this case a thin oxide film may be formed on the surface of the amorphous silicon film since when the solution is directly coated on the silicon film, the solution is repelled therefrom. Thermal oxidation, oxidation by an oxidant such as hydrogen peroxide, oxidation by ultraviolet ray irradiation or the like may be used in forming the oxide film.
It is also useful to add a surfactant to a solution containing a catalyst element instead of forming an oxide film. The addition is for enhancing adherence and controlling adsorptiveness to a face to be coated. The surfactant may previously be coated on the face to be coated. As the surfactant a hydrocarbon chain basically containing approximately 10 through 20 carbon atoms as a hydrophobic radical may be used.
For example, as the surfactant there is a mixed solution of hydrofluoric acid, a solution of ammonium fluoride and water containing at least one material selected from the group consisting of surfactants comprising a salt of a fatty acid and carboxylic acid, a salt of a fatty acid and a carboxylic acid, a fatty acid amine and an aliphatic alcohol. The fatty acid may be designated by Cn H2n+1COOH (n is an integer of 5-11). A salt of a fatty acid may be designated by Cn H2n+1CONH3R (n is an integer of 5-11. R designates a hydrogen atom or an alkyl group having a carbon number of 5-10). A fatty acid amine may be designated by a general formula of CmH2m+1NH2 (m is an integer of 7-14). An aliphatic alcohol may be designated by a general formula of Cn H2n+1OH (n is an integer of 6-12).
Specific examples of the surfactants are shown in the following Tables 1 through 3. The following surfactants have an operation of dispersing a metal element when it adheres to the surface of the amorphous silicon film.
When nickel is used as a catalyst and the nickel is included in a poler solvent, the nickel is introduced in the form of a nickel compound. Representative nickel compounds are selected from nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate, nickel acetylacetonato, nickel 4-cyclohexylbutyrate, nickel oxide and nickel hydroxide.
Further, when a simple substance of nickel is used as a catalyst element it is necessary to form a solution by solving nickel in an acid.
Solvents including a catalyst element may be selected from nonpolar solvents of benzene, toluene, xylene, carbon tetrachloride, chloroform and ether. In this case nickel is introduced in the form of a nickel compound. Representative nickel compounds may include nickel acetylacetonato and nickel 2-ethylhexanate.
Although the above-mentioned example uses solutions in which nickel as a catalyst element is completely dissolved, an emulsion in which a powder comprising a single substance of nickel or a nickel compound is uniformly dispersed in a dispersion medium may be used even if nickel is not completely dissolved. Or, a solution for forming an oxide film may be used. Such a solution includes OCD (Ohka Diffusion Source) of Tokyo Ohka Kogyo K.K. In using OCD solution, it is coated on a face to be processed and baked at approximately 200xc2x0 C. by which a silicon oxide film can simply be formed. Further, the solution can be utilized in the present invention since impurities can be added thereto freely.
The same is applicable in case where a material other than nickel is used as a catalyst element.
It is preferable that the amount of the catalyst element included in the solution is 0.1 ppm through 200 ppm in a nickel amount with respect to a solution as a general tendency, preferably 1 ppm through 50 ppm (in weight), although depending on the kind of solution. This is a value determined in view of a nickel concentration in a film in which the crystallization has been finished or chemical resistance (for example, hydrofluoric acid resistance).
It has been revealed that the large off current of TFT made by using a crystallized silicon film is caused by an excessive presence of a catalyst element used in crystallization in crystals and the large dispersion of the off current is due to segregation of the catalyst element. Therefore, the off current can be reduced if the catalyst element is excluded from the silicon crystals to the outside after the crystallization step and a low concentration of the catalyst element can avoid the segregation.
Originally, the catalyst element cannot stably be present in silicon crystals and is excluded in a natural fashion. However, strong blocking layers (barrier) are actually formed on top and bottom of the silicon film and accordingly, the catalyst element is contained inside and is segregated on grain boundaries.
One of the characteristics of the present invention is continuous formation of an underlayer of the silicon oxide film and the amorphous silicon film. That is, no foreign layer is formed between the underlayer of the silicon oxide film and the amorphous silicon film by moisture, carbon dioxide and the like in the atmosphere. In the present invention the underlayer of the silicon oxide film and the amorphous silicon film are deposited at a low temperature of 450xc2x0 C. or less and therefore, the oxide silicon film is very soft and the catalyst element excluded from the amorphous silicon film is swiftly incorporated in the silicon oxide film.
Conversely, almost no catalyst element is absorbed in the film if the underlayer is made of silicon nitride having a strong blocking action. Further, even with the silicon oxide film, if it is treated at a temperature exceeding 450xc2x0 C., silicon oxide is solidified and absorption of the catalyst element is hindered.
At the initial stage of crystallization an interface between silicon oxide and silicon is in an indefinite state (a state in which an interface between different substances constituted by stoichiometric compositions is not recognized). However, the interface becomes definite with the progress of crystallization. Much of the catalyst element is present at the front end of crystallization and is moved with the progress of crystallization. Therefore, much of the catalyst element is absorbed in the silicon oxide film in the state in which all the silicon film has finally been crystallized.
The initially soft silicon oxide film is sufficiently solidified by an annealing step and almost no catalyst element which has been absorbed in the silicon oxide film does not retrogress into the crystal silicon film. Further, the trap level etc. can sufficiently be reduced. Therefore, no problem is caused with regard to reliability in the successive element formation.